mac80211: remove unused variable in ieee80211_parse_ch_switch_ie()

The ht_oper variable is assigned a value, but never used in
ieee80211_parse_ch_switch_ie().  Remove it.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>